This invention relates to separators for use in electrochemical cells and, in particular, for use in electrochemical cells having a zinc electrode.
As is known, the cycle life of alkaline batteries employing zinc electrodes such as, for example, nickel-zinc, silver-zinc and manganese dioxide-zinc, is limited by a number of factors. One primary factor is shorting of the battery electrodes which results from the growth of zinc dendrites. These dendrites produce tree-like conductive branches which eventually penetrate the battery separator to provide a conductive path between positive and negative electrodes, thereby shorting same. Another factor limiting battery life is so called "shape change" of the zinc electrode. Shape change is the phenomenon of movement of the zinc active material from the electrode edges to the electrode center, thereby significantly reducing the electrode surface area available for electrochemical reaction.
As is known, the degree of dendritic growth and the degree of shape change occurring in an alkaline battery are a function of the battery separator. Conventional battery separators typically comprise one or more layers which are selected to be compatible with the cell electrolyte. These layers are also selected to be sufficiently permeable to allow the passage of electrolyte ions, but not so highly permeable as to readily permit the passage of other materials, e.g., electrode derived conductive substances. Separators of this type usually contain at least one layer of membranous material (i.e., a material having a permeability of about 5.times.10.sup.-3 mole/(min.times.in.sup.2) or less. A typical membranous material might be cellophane. Another type of membranous material might be the material disclosed in U.S. Pat. No. 3,351,495. Such membranous materials are found to significantly inhibit dendritic growth but are not as effective in inhibiting electrode shape change which still can occur and eventually result in battery failure.
A further separator construction has been proposed wherein the separator comprises a blend of a polyamide (e.g., Elvamides), a significantly lower pore size polymeric constituent (e.g., polyethylene oxide) and a zinc reactive material (e.g., cerium dioxide). In this separator, while the polymeric constituent acts to inhibit dendritic growth akin to the above-described membranous materials, the polyamide constituent provides sufficient permeability to significantly retard shape change. The resultant separator is thus said to prevent both zinc dendritic growth and electrode shape change.
While the separators described above have provided a degree of extended battery life, research is still being conducted for separators which can extend battery life still further.
It is an object of the present invention to provide a separator for an alkaline battery which enables longer cycle life than heretofore obtainable.
It is further object of the present invention to provide an alkaline battery separator which further retards dendritic growth and electrode shape change.